1. Field of the Invention
The present invention relates to a color image forming coupler which is suitable for use in color photography. More particularly, it relates to a two-equivalent type magenta color image forming coupler having a novel coupling releasable group.
2. Description of the Prior Art
In a color photographic process using a silver halide light-sensitive material, a color image forming coupler which is present in a developer solution or incorporated into a photographic emulsion layer or another hydrophilic colloid layer reacts with an oxidation product of a color developing agent formed by development of an exposed silver halide emulsion layer to form a non-diffusible color image. Most conventional couplers are four-equivalent couplers, that is, theoretically, they stoichiometrically require four mols of exposed silver halide as an oxidizing agent for formation of one mol of a dye. Incorporation of a large amount of silver halide in a light-sensitive layer is disadvantageous because the sharpness of the images formed is deteriorated due to the increase of light scattering in the emulsion layer or because the rate of processing the light-sensitive material is reduced due to the increase in the thickness of the emulsion layer. Further, there is a disadvantage in the formation of dyes using these four-equivalent couplers since a strong oxidizing agent must be employed in the subsequent processing step, because the formation of the dyes is not completed in a color development bath.
In order to overcome these disadvantages, a two-equivalent coupler, that is, a coupler which requires only two mols of exposed silver halide for forming one mol of a dye, has been proposed.
A two-equivalent coupler has a structure in which one hydrogen atom at the coupling position, for example, the paraposition to the hydroxy group of a phenolic, the active methylene group at the 4-position of a 5-pyrazolone or the active methylene group of an acylacetanilide, is replaced by a group capable of being released upon coupling. Known examples of these releasable groups are a sulfonamido group, an imido group, a sulfonyl group, an aryloxy group, a thiocyano group, a carbamoyl group, an aminosulfonyloxy group, a sulfinyloxy group, and the like. However, the known two-equivalent couplers having these releasable groups are not always satisfactory, because they have disadvantages with respect to coupling reactivity, dispersibility, color fog, ease of synthesis, stability of the color image formed, and the like.
A two-equivalent type coupler having a 5-membered heterocyclic group which contains at least one nitrogen atom and an ethylenic double bond as a releasable group as described in Japanese Patent Application (OPI) No. 34232/1975 is also known. The 5-membered ring can contain at most three nitrogen atoms and in such a case the 5-membered ring having three nitrogen atoms is a 1,2,3-triazole ring. However, when a two-equivalent coupler having such a 1,2,3-triazole ring as a releasable group is incorporated into a light-sensitive material, the stability of the light-sensitive material during storage is not satisfactory.
Furthermore, of two-equivalent couplers a development inhibitor releasing coupler, the so-called DIR coupler which is used for improving the sharpness of the color image due to edge effects within a layer, reducing the graininess of the color image or achieving color corrections due to the interlayer effects, is known. However, DIR couplers generally give rise to a decrease in the maximum color density and the gradation.